class Bird
	def tweet(bird_type)
		bird_type.tweet
	end
end

class Cardinal < Bird
	def tweet
		puts "Tweet tweet"
	end
end

class Parrot < Bird
	def tweet
		puts "Squawk"
	end
end

generic_bird = Bird.new
generic_bird.tweet(Cardinal.new)
generic_bird.tweet(Parrot.new)